There are continuous attempts in the field of paper industry to find ways of reducing costs when producing paper or board without worsening properties, such as strength, of the paper or board. The costs have been reduced for example by increasing filler content of a paper or board. When increasing the filler content, the amount of fibers in the paper or board can be reduced. On the other hand, large amount of fillers in the paper or board decreases its strength.
The decrease in strength can be compensated by improving the fiber bonding properties between the fibers in the paper or board, thus maintaining the strength. The predominant treatment for improving paper or board strength has been to add a strength additive, such as starch (cationic starch), to the stock (also called furnish) prior to the sheet forming operation. Molecules of cationic starch that have been added to the stock can adhere to the naturally anionic pulp fibers by electrostatic attraction and thus be retained in the wet fiber mat and remain in the final paper or board.
By adding large amounts of cationic starch to the stock, in order to achieve high paper strength, problems occur. The cationic starch molecules tend to saturate the anionic charge on the cellulose fibers, thus setting a limit to the amount of cationic starch which can be added to the pulp slurry. If an excess of cationic starch is added, only a portion of the starch added will be retained in the sheet, and the rest will circulate in the paper or board machine white water system. Moreover, fibers which are made cationic by excessive cationic starch addition will not be able to absorb other cationic additives which are commonly added to the pulp slurry, for example sizing agents and retention aids. Large amounts of starch often cause also problems with runnability and foaming during the production process.
Addition of microfibrillated cellulose (MFC), also known as nanocellulose, to a paper or board will increase the strength of the product. This is likely due to improved fiber bonding.
Microfibrillated cellulose is a material typically made from wood cellulose fibers. It can also be made from microbial sources, agricultural fibers, dissolved cellulose or CMC etc. In microfibrillated cellulose the individual microfibrils have been partly or totally detached from each other.
WO 2011/068457 discloses a process for producing a paper or board product which contains microfibrillated cellulose. The process comprises the steps: providing a furnish comprising fibers, adding starch to the furnish, adding microfibrillated cellulose to the furnish, and conducting the furnish to a wire in order to form a web, wherein the starch and microfibrillated cellulose are added separately to the furnish. The furnish comprises starch in an amount of 2-15% by weight and microfibrillated cellulose in an amount of 1-15% by weight.
Microfibrillated cellulose has a very high water binding capacity and it is thus very difficult to reduce the water content of a slurry comprising microfibrillated cellulose. High water content of a slurry comprising microfibrillated cellulose also prevents usage of microfibrillated cellulose in many different applications where microfibrillated cellulose with high solids would be required.
Use of microfibrillated cellulose in paper and board applications will produce denser paper structure, but with worse dewatering properties. Drainage time increases as a function of microfibrillated cellulose amount.
Thus, there is a need for an improved and more efficient process for producing paper or board from microfibrillated cellulose containing stocks having improved dewatering properties.